internal/pkg: optionally named static file

These are generally for generating configuration files or build scripts, naming them is quite useful.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/file_test.go

@@ -15,7 +15,7 @@ func TestFile(t *testing.T) {
 			c.SetStrict(true)
 
 			cureMany(t, c, []cureStep{
-				{"short", pkg.NewFile([]byte{0}), base.Append(
+				{"short", pkg.NewFile("null", []byte{0}), base.Append(
 					"identifier",
 					"lIx_W4M7tVOcQ8jh08EJOfXf4brRmkEEjvUa7c17vVUzlmtUxlhhrgqmc9aZhjbn",
 				), pkg.MustDecode(